Semiconductor switching devices, such as transistors, diodes, MOSFETs and thyristors, are well known in the art. These devices have a semiconductor structure, such as silicon, GaAs and the like, having regions or layers with implanted dopants (impurities) at density levels determined by the type of semiconductor device and its desired application. The type of dopant implanted dictates whether the doped layer is a positively doped layer (a p-layer) or a negatively doped layer (an n-layer). The number and arrangement of layers varies depending on the type of device.
For example, a semiconductor diode includes adjacent p and n layers, an n-p-n semiconductor transistor includes a single p-layer sandwiched between two n-layers, and a p-n-p transistor includes a single n-layer sandwiched between two p-layers. A thyristor, on the other hand, is a 4-layer semiconductor structure with alternate n and p layers arranged in the order n-p-n-p. Thyristors are commonly used as high power switching devices which are capable of operating in high current environments.
To configure a thyristor as a high power switching device, the n-p-n-p layer semiconductor structure is fabricated using any known techniques. Metal layers are then attached to opposite sides of the structure by, for example, sputtering, vacuum-film deposition, or any other known process. Hence, the metal layer attached to the n-layer side functions as a cathode electrode of the device, while the metal layer attached to the p-layer side functions as an anode electrode. Gate structures, which control the activation of the switch, are also formed, for example, on the cathode side of the device.
The device including the anode electrode, cathode electrode and gate structures can then be mounted in a suitable housing, such as an insulative housing having anode and cathode terminals which are coupled to the anode electrode and cathode electrode, respectively, of the device. The housing also includes gating terminals which are coupled to the gate structures of the device.
The assembled power switching device can then be employed, for example, as a switch that is controlled to regulate high ampere current being provided to a load. In this type of application, the cathode or anode of the power switching device can be coupled to a power supply, and the anode or cathode can be coupled to the load. When a control voltage is applied to the gate terminal, current carriers (i.e., electron-hole pairs) are generated in the semiconductor structure which initiates regenerative carrier production by the 2 transistor model for this class of device. This permits current provided by the power supply to flow through the switch to the load. After the control current is removed from the gate terminal, and the load current falls below some critical value (holding current), generation of current carriers ceases, and the device recovers to its "off state" in which the center p-n junction is reversed biased and prevents passage of current from the power supply to the load.
Several problems exist with power switching circuits of this type which employ conventional gate structures. In particular, the gating structures typically occupy as much as 45% of the area of the cathode surface of the device. Because load current must flow through the actual cathode electrode/semiconductor contact area, load current is forced to redistribute around the area of gate and cathode contacts at the cathode surface. Because ohmic heating limits the current density in the semiconductor, the amount of current that the device can accommodate is significantly limited by the gating structure. Accordingly, the area of the semiconductor structure must be made larger to accommodate higher current levels, thus increasing the overall size of the switch. However there is a practical limit to the size of semiconductor wafers that can be manufactured, typically a diameter of less than 155 mm. Thus, these 4 layer switches have limited current carrying capability.
Furthermore, the redistribution of current flowing through the device around the gate regions creates non-uniform current distributions in the device such as current filaments. These non-uniform current distributions increase the probability of thermal runaway, which can shorten the lifespan of the device because of localized heating and damage.
Also, in semiconductor switches having conventional gate structures, a plasma spreading phenomenon occurs from the gate attachments to the p-type base semiconductor structure, which limits the rate of rise of current (di/dt) through the device. Although more complicated gate geometries have been developed which minimize the adverse affect of the plasma spreading phenomenon, such complicated gate structures increase the overall complexity of the device.
In extreme applications (e.g., high di/dt, high current, high charge transfer), the conventional gate structures also suffer from a high incidence of failure. For example, in the n-p-n-p structure described above, gate structures are applied to portions of the p-type base structure which has been "pulled up" through the n-layer cathode surface of the structure. This arrangement creates adjacent areas in the semiconductor structure which are electrically isolated from each other. When a large amount of drive current is applied to the gates, transient potential differences can exist between the gates and the cathode surface adjacent the gates. This difference in potential can create surface discharges in the structure which eventually short the adjacent electrically isolated areas together and render the device inoperable.
To overcome the problems associated with conventional gated power switching devices, a laser activated silicon switch is described in a publication by L. R. Lowrey entitled "Optically Activated Switch", Westinghouse Research and Development Center, Pittsburgh, Pa., April 1978. The laser activated silicon switch employs a laser, such as a high power Nd-YAG laser, which operates as the gating device in place of a conventional gating structure. This laser emits laser light which is absorbed by the semiconductor structure of the type described above. The absorbed laser light (photons) excites the semiconductor structure to generate electron-hole pair current carriers, which permit current to flow through the semiconductor structure. Hence, the switch is controlled to operate in active and inactive modes by controlling the laser.
Another type of laser activated switch is described in a publication by A. Rosen and P. J. Stabile entitled "Targetted Applications for Laser Activated Switching", David Sarnoff Research Center, Princeton, N.J. This type of laser activated switch includes a plurality of PIN diode arrays which are connected in series to receive electrical input signals. A laser array is positioned above the PIN diode arrays to irradiate light that is absorbed by the semiconductor structures of the PIN diodes. The absorbed light causes the semiconductor structures to generate current carriers (electron-hole pairs) which allow current to flow through the diodes. Hence, the laser array is controlled to control the flow of the electrical input signals through the diode arrays.
Other types of laser activated switches are described is U.S. Pat. Nos. 5,017,991, 4,974,047, 4,779,126, 4,500,164, 4,368,481 and 4,016,592. However, these devices are unsuitable for high di/dt applications.
Although laser activated switches are known in the art as demonstrated above, a continuing need exists for improved laser activated semiconductor switches. For example, a need exists for a laser activated semiconductor switching device having a semiconductor structure and laser array which are packaged together as a single unit, to therefore reduce the size of the device. A continuing need also exists for a laser activated semiconductor switching device capable of handling very high current and a very high rate of current rise, so that the switching device can be employed as a high power switching device. It is also desirable for the switching device to have improved current flow efficiency and low failure rate.